1. Field of the Invention
The field of the present invention is optical systems, particularly optical systems which include at least one segmented reflective surface and devices for aligning the segments of the reflective surface.
2. Background
Imaging systems are frequently deployed on either airborne or orbiting platforms for intelligence gathering purposes. Over the years, image processing techniques have been developed which provide the intelligence provided through image analysis of terrestrial scenes. Most, if not all, of the image processing techniques benefit from 1) an increased image resolution and 2) an increased signal-to-noise (SNR) ratio. Both of these factors are improved as the size of the entrance aperture for the imaging system, which is typically defined by the size of the primary mirror, is increased. Unfortunately, the size of the entrance aperture is often constrained by the physical size of components that can be lifted into orbit using modern launch vehicle technology.
One solution to the physical size limitation is transporting the primary mirror in segments and assembling those segments at the platform to create a large primary mirror. A segmented mirror, however, introduces its own set of complications. Chief among the possible complications is misalignment of the segments, which may have many sources, including vibrations induced by platform motion and operation of system components such as gyros and coolers. To achieve proper functionality of the imaging system, the mirror segments are aligned in both “piston” and “tilt”. “Piston alignment”, also referred to herein as the “piston position”, is the alignment of each segment along the z-axis, which is generally parallel to the optical axis of the mirror. A segment having an incorrect piston position is referred to as having “piston misalignment” or “piston error”. “Tilt alignment”, also referred to herein as the “tilt position”, is the alignment of each segment relative to the x-y plane, which is orthogonal to the z-axis of each segment. A segment having an incorrect tilt position is referred to as having “tilt misalignment” or “tilt error”. Both tilt and piston misalignments are highly undesirable because they introduce aberrations in the primary mirror, thus degrading the overall image quality achievable by the system.
Conventional adaptive optics have previously been designed to correct aberrations in an optical system and generally rely upon measuring the wavefront of light passing through the optics of the system using an unresolved bright light source. For airborne optical systems, this type of adaptive optics often proves unworkable because the scenes being imaged generally do not include such a bright light source. This difficulty may be avoided altogether by adoption of a different approach to correcting the optical aberrations.